This invention relates in general to anti-lock brake systems and in particular to a circuit and method for modulation of the voltage utilized to actuate solenoid valves in an anti-lock brake system and for verification of correct functioning of electrical components in the anti-lock brake system.
An Anti-lock Brake System (ABS) is often included as standard equipment on new vehicles. When actuated, the ABS is operative to modulate the pressure applied to some or all of the vehicle wheel brakes. A typical ABS includes a hydraulic control valve which has plurality of solenoid valves mounted within a control valve body. The control valve body is connected to the vehicle hydraulic brake system. The valve body also includes an accumulator for the temporary storage of brake fluid during an anti-lock braking cycle.
A separate hydraulic source, such as a motor driven pump, is usually included in the ABS for reapplying hydraulic pressure to the controlled wheels during an ABS braking cycle. Alternately, the pump can return brake fluid from the accumulator to the vehicle master brake cylinder during an ABS braking cycle. The pump is typically included within the control valve body while the pump motor is mounted upon the exterior of the control valve body. The pump motor is usually a direct current motor which operates from the vehicle power supply.
An ABS further includes an electronic control unit which has a microprocessor. The electronic control unit is usually attached to the hydraulic control valve. The microprocessor is electrically coupled to the pump motor, a plurality of solenoid coils associated with the solenoid valves, and wheel speed sensors for monitoring the speed and deceleration of the controlled wheels. The microprocessor includes a memory portion which stores an ABS control algorithm. The ABS control algorithm comprises a set of instructions for the microprocessor which control the operation of the ABS. The instructions typically include a set of operational checks which are run during vehicle start up to assure that the ABS is functional. The control program also includes subroutines for monitoring the vehicle operation to detect a potential lock-up of the controlled wheel brakes and for the actual operation of the ABS during an anti-lock braking cycle.
During vehicle operation, the microprocessor in the ABS control unit continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to initiate an ABS braking cycle. During the ABS braking cycle, the microprocessor actuates the pump motor and selectively operates the solenoid valves in the control valve to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the vehicle operator.
This invention relates to a circuit and method for modulation of the voltage utilized to actuate solenoid valves in an anti-lock brake system and for verification of correct functioning of electrical components in the anti-lock brake system.
As described above, a typical ABS modulates the pressure applied to the controlled wheel brakes by cyclically opening and closing solenoid valves. In prior art systems, such valves have been digital, having two operating positions, open or closed. The valve is quickly switched between the two positions. Such switching can cause undesirable acoustic noise, particularly at low vehicle speeds where the noise is more noticeable. Additionally, the solenoid coil may be driven into saturation, causing a delay in valve transition when the solenoid current is interrupted. Accordingly, it would be desirable to provide a better control of the solenoid valves while also reducing the hydraulic system noise caused by operation of the valves.
The present invention contemplates a control unit for a solenoid valve comprising an electronic switch adapted to be connected to a solenoid valve coil for controlling the flow of an electric current therethrough. The switch includes a control port and is responsive to a control signal applied to the control port to transition between conducting and non-conducting stages. The control unit also includes a signal generator connected to the control port of said electronic switch. The signal generator is operative to generate a control signal which can be a pulse train signal having either a variable duty cycle and a constant frequency or a variable frequency and a constant duty cycle.
In the preferred embodiment, the control signal has a variable duty cycle. Also, the solenoid valve coil is connected between a first terminal of the switch and a power supply while a second terminal of the switch is connected to ground. The current flowing through the coil is a function of the control signal duty cycle. It is further contemplated that the control signal generator is included within a microprocessor.
The control unit also includes a voltage divider connected across the electronic switch. The voltage divider includes a center tap connected to a feedback port of the microprocessor with the microprocessor being responsive to a voltage appearing at the center tap to confirm the operation of the electronic switch.
It is further contemplated that the control unit includes a diode having a cathode connected to the end of the solenoid coil connected to the power supply and a Zener diode having an anode connected to an anode of the diode. The Zener diode also has a cathode connected to the junction of the coil and the electronic switch. The diode and Zener diode are operative to provide a discharge path for the coil current when the switch changes from a conducting to a non-conducting state.
The control unit also includes a feedback resistor connected between the junction of the diode and the Zener diode and the center tap of the voltage divider.
The voltage appearing at the center tap of the voltage divider is a function of the voltage appearing across the solenoid coil and the microprocessor is responsive thereto to confirm the operation of the solenoid coil. The control unit is included in an antilock brake system or a traction control system.
The invention also contemplates a method for controlling the current flowing through a solenoid valve coil which includes providing an electronic switch connected to the valve coil, the switch having a control port and being responsive to a control voltage applied to the control port to change between conducting and non-conducting states. A pulse width modulated control voltage having a constant frequency and variable duty cycle is applied to the control port of the switch. The control voltage has an initial duty cycle having a first predetermined value, the initial duty cycle being sufficient to cause the valve armature to begin to move within the valve from a deactuated position to an actuated position. The duty cycle is reduced to a duty cycle having a second predetermined value upon the valve armature reaching a desired position within the solenoid valve, the second predetermined value being less than the first predetermined value. The second predetermined value is sufficient to provide a coil current which will hold the valve armature in the desired position.
The method further includes decreasing the duty cycle to third predetermined value which is less than the second predetermined value to cause the valve armature to begin to move back toward the deactuated position. The method also can include increasing the duty cycle to a fourth predetermined value as the valve armature approaches the deactuated position to slow the return of the armature to the deactuated position. The fourth predetermined value for the duty cycle is between the second and third predetermined values.
The invention further contemplates a method for monitoring the operation of an electronic switch connected between a solenoid valve coil and ground which includes providing a device for monitoring the voltage across the electronic switch. When the electronic switch is closed, the voltage across the electronic switch is sampled and compared to a first threshold voltage. If the sampled voltage is greater than the first threshold voltage, a warning device is actuated.
When the electronic switch is open, the voltage across the electronic switch is sampled and compared to a second threshold voltage. If the sampled voltage is less than the second threshold voltage, the warning device is actuated. Additionally, the sampled voltage can be compared to a third threshold voltage, which is greater than the second threshold voltage, and the warning device actuated if the sampled coil voltage is greater than the third threshold voltage.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.